This invention relates to improved processes for the production of acicular, ferromagnetic iron oxides of highly uniform particle size and magnetic characteristics suitable for use in magnetic recording medium.
In recent years, a demand for a high quality of magnetic recording media including magnetic tape, magnetic disc, et al., has been more and more increasing with the progress of miniaturization and lightening of reproducing apparatuses for magnetic recording. To be specific, magnetic recording media of higher bit density, higher output, higher sensitivity and improved frequency characteristics et al., has been requested. These magnetic properties of magnetic recording media are mainly influenced by their residual magnetic flux density Br, while the residual magnetic flux density Br is strongly influenced by properties of the magnetic powder such as their dispersiveness in a vehicle, their orientation and packing property in a coating medium. Iron oxide needles such as acicular magnetite powder and acicular maghemite powder used nowadays as a main magnetic recording material is usually prepared in such a manner that acicular .alpha.--FeO(OH) is reduced in a stream of a reducing gas such as H.sub.2 at a temperature of 300.degree. C.-400.degree. C. to produce acicular magnetite powder, or thereafter the produced acicular magnetite powder is re-oxidized at a temperature of 200.degree. C.-300.degree. C. in the air to produce acicular maghemite powder.
The starting acicular .alpha.--FeO(OH) is prepared by, as a typical manner, adding more than equivalent of an alkaline aqueous solution to an aqueous solution of ferrous salt to produce an aqueous suspension containing precipitated Fe(OH).sub.2, and thereafter oxidizing the obtained suspension at a pH of more than 11 and at a temperature of less than 80.degree. C. The acicular .alpha.--FeO(OH) thus obtained is in a form of needles having a length of about 0.5-1.5.mu.m while dendoroidal particles are inevitably included therein, and further their particle size can not be said to be uniform.
These difficulties are brought about in the conventional process by reason that:
A. Ferrous hydroxide floccules, the precursor of the acicular .alpha.--FeO(OH) needles, contained in a flocculated form in the iron (II) hydroxide suspension originally have non-uniform floc-size. Further, it should be noted that iron (II) hydroxide particles per se forming each floccule also have non-uniform particle size which contributes to the formation of the acicular .alpha.--FeO(OH) needles having non-uniform particle size.
B. In general, the reaction for producing the acicular .alpha.--FeO(OH) needles comprises two reaction steps, namely, the first generation step of .alpha.--FeO(OH) nuclei and the second growth step of the .alpha.--FeO(OH) nuclei to the needles through the contact of Fe(OH).sub.2 with the dissolved oxygen.
However, as the contact reaction of Fe(OH).sub.2 with the dissolved oxygen is performed only partially and unevenly, the .alpha.--FeO(OH) nuclei generation step and the growth step thereof to the needles occur simultaneously and new nuclei of the .alpha.--FeO(OH) are repeatedly generated until the production of the .alpha.--FeO(OH) needles is completed.
Thus, owing to these facts, it is considered that the acicular .alpha.--FeO(OH) needles having irregular particle sizes and including dendroidal particles are inevitably produced.
C. The concentration of reactant Fe(II) in the aqueous suspension according to the conventional process is usually about 0.2 mol/l while the formation of the acicular .alpha.--FeO(OH) needles requires long reaction times. That is, in the conventional process, the particle size of the resultant .alpha.--FeO(OH) needles is not uniform even at a Fe(II) concentration of as low as about 0.2 mol/l. On the other hand, it is well known that as the concentration of the reactant Fe(II) becomes larger, the more dendroidal particles come to be mixed in. Accordingly, the acicular magnetite needles or the acicular maghemite needles prepared by the reduction and reoxidation of the starting acicular .alpha.--FeO(OH) prepared by the conventional process is also uneven in particle size thereof and contaminated with the dendroidal particles. Of course, the magnetic recording medium made of such magnetic material as low quality in powder characteristics can not provide high Br values. Under the foregoing situations in the art, many efforts have been made in the prior art for preparing starting acicular .alpha.--FeO(OH) of high quality having uniform particle size and not contaminated with dendroidal particles.
For example, in Japanese Patent Laying Open No. 57397/74 disclosed is a process for preparing .alpha.--FeO(OH) particles in which an aqueous solution of ferrous salts is allowed to react with an alkaline aqueous solution to produce a precipitated Fe(OH).sub.2 suspension which thereafter is stirred by means of a stirrer or supersonic waves for several hours under a non-oxidative condition to produce colloidal, white-milky and finely dispersed Fe(OH).sub.2 precipitates and then to form .alpha.--FeO(OH) needles by an oxidative reaction with the air or an oxygen-containing gas.
In Japanese Patent Laying Open No. 867695/76 disclosed is a process in which a white precipitate of Fe(OH).sub.2 in a gel form is oxidized while an oxygen-containing gas is being introduced in an amount of less than 100 cc/min. calculated as the ratio of pure oxygen per 0.5 molar Fe(OH).sub.2. In this process, the white precipitate of Fe(OH).sub.2 in a gel form is oxidized very slowly and whereby it is intended to obtain acicular .alpha.--FeO(OH) including less dendroidal particles and having more uniform particle size.
Further, in Japanese Patent Laying Open No. 57996/75 disclosed is a process for preparing acicular .alpha.--FeO(OH) particles of superior acicularity and having uniform particle size, in which seeds of .alpha.--FeO(OH) are formed at a pH of 2-8 and under the presence of about 0.1-4% by weight of zinc ion and about 0.1-2% by weight of phosphate ion based on the amount of .alpha.--FeO(OH) to be precipitated.
On the other hand, in U.S. Pat. No. 3,947,502 and Japanese Patent Laying Open Nos. 83100/73 and 41299/74 disclosed is a process for improving stabilization of the acicular .alpha.--FeO(OH) against sintering by coating the .alpha.--FeO(OH) needles with SiO.sub.2, which includes the step of effecting the oxidation of the iron (II) hydroxide suspension at a pH of at least 13, before the oxidation is more than 95% complete but after the oxidation is more than 50% complete SiO.sub.2 being present in the suspension in the form of a sol or soluble salt, the resulting iron (III) oxide hydroxide having SiO.sub.2 deposited thereon.
However, although the .alpha.--FeO(OH) needles having SiO.sub.2 deposited thereon prepared according to the above process are prevented from sintering during the heat treatment thereof, the uniformity of particle size of the needles themselves is not in the least improved.
Accordingly, an object of this invention is to provide an improved process for preparing iron oxide needles such as acicular magnetite and maghemite having highly uniform particle size and being not contaminated with dendroidal particles, and suitable for use in magnetic recording medium.
Another object of this invention is to provide an improved process for preparing iron oxide needles having uniform particle size and also large length-to-width ratio to be required.
These and other objects of this invention will become more apparent from the following description.
The foregoing and other objects of this invention will be accomplished by the process of this invention in which the formation of acicular iron (III) oxide hydroxide (.alpha.--FeO(OH)) as starting material is carried out by previously adding a critical amount of water-soluble silicate and preferably water-soluble zinc compound into an iron (II) hydroxide (Fe(OH).sub.2) suspension at a pH of at least 11 and thereafter oxidizing the iron (II) hydroxide suspension with an oxygen-containing gas.